1. Field of the Invention
This invention relates to a process for producing a branched or cross-linked polyacetal resin which excels in formability, particularly blow moldability, physical properties, and surface attributes.
2. Description of the Prior Art
The polyacetal resins exhibit mechanical quality, chemical resistance, and slidability in highly harmonious balance and enjoy ease of formation and, owing to these good features, find extensive utility as typical engineering plastics chiefly in electric/electronic parts, automobile parts, and other various machine parts. Most of them come in the form of injection molded articles.
In recent years, the expectation that the polyacetal resins, by virtue of their excellent resistance to chemicals, to organic solvents in particular, will find utility in applications to automobile fuel tanks, hollow components in engine rooms, or various containers has been gaining in prominence. The blow molding technique is generally used as the efficient means for producing such blow molded articles as mentioned above. It is held that for a resin to be effectively blow molded or extrusion molded, particularly to be blow molded, the exaltation of the melt tension of this resin generally constitutes itself an essential condition to an extent of protecting a parison of the resin against drawdown or preventing a blow molded article of the resin from sustaining a rupture or forming an uneven wall thickness while in process of production. For the sake of exalting the melt tension of a resin, measures for giving the resin a heightened molecular weight are generally adopted. In the case of such general-purpose resins as polyethylene which are used for blow molding, for example, those of the type having extremely high molecular weights are widely used.
The polyacetal resins generally excel in injection moldability. They, however, are blow molded with extreme difficulty because they exhibit such low melt tension to blow molding as suffers their parisons to induce the phenomenon of drawdown. Attempts to improve the blow moldability of a resin by devising a method of polymerization for giving the resin a heightened molecular weight are occasionally found as in JP-A 5-301271, for example. Since the polyacetal resins which have been made to acquire generously heightened molecular weights are markedly degraded in flowability, they are still deficient in blow moldability in spite of the improvement in liberation from the phenomenon of drawdown. The blow molded articles made of polyacetal resins having such high molecular weights and high degrees of viscosity as mentioned above are at a disadvantage in losing such impact resisting properties as surface impact strength ascribable to the orientation of resin.
In contrast, the polyacetal resins which have been made to form a branched or cross-linked structure tend to exhibit low degrees of viscosity in a state of high shear for their high molecular weights. For these polyacetal resins to reconcile an ample melt tension with flowability, however, they are required to add to their numbers of branches or cross-links. This addition, however, brings about the problem that the resins are deficient in impact resistance, the shaped articles of the resins are deficient in smoothness of inner surface, and the finished products of shaped articles are deficient in commercial value.
JP-A 7-195496 discloses compositions which are formed of polyacetal resin and thermoplastic polyurethane resin. These compositions are still improper because they seriously impair the inherent properties of polyacetal resin and, moreover, fail to exhibit stable productivity.